📚 Organic Reactions – Mechanisms | 有机反应——机理
Understanding organic reaction mechanisms is central to A-Level Chemistry. A mechanism describes the step-by-step pathway by which bonds are broken and formed, showing the movement of electrons and the intermediates involved. Mastering these pathways allows you to predict products, explain selectivity, and appreciate the logic of organic synthesis.
理解有机反应机理是A-Level化学的核心。机理描述了化学键断裂和形成的分步路径，展示电子的移动过程以及中间体的参与。掌握这些路径能够帮助你预测产物、解释选择性，并领会有机合成的逻辑。
1. Introduction to Reaction Mechanisms | 反应机理简介
A reaction mechanism illustrates the sequence of bond breaking and bond making, using curly arrows to represent the flow of electrons. It often includes reactive intermediates such as carbocations, carbanions, or free radicals. A full mechanism accounts for all starting materials, reagents, and products, explaining how the reaction proceeds at the molecular level.
反应机理展示化学键断裂和形成的过程，使用弯箭头表示电子的流动。它通常包括活泼中间体，如碳正离子、碳负离子或自由基。完整的机理解释了起始原料、试剂和产物如何在分子水平上一步步转化。
In A-Level questions, you may be asked to draw or describe mechanisms, identify rate-determining steps, or explain the role of catalysts. Always show partial charges (δ⁺, δ⁻) and lone pairs where relevant.
在A-Level试题中，你可能需要画出或描述机理、识别决速步或解释催化剂的作用。只要相关，就应标出部分电荷（δ⁺, δ⁻）和孤对电子。
2. Bond Fission: Homolytic and Heterolytic | 键断裂：均裂与异裂
Bond fission is the process of breaking a covalent bond. There are two types: homolytic fission and heterolytic fission. In homolytic fission, the bond breaks evenly so that each atom receives one electron from the shared pair, generating two free radicals (e.g. Cl–Cl → 2 Cl•). Radicals are highly reactive species with an unpaired electron.
键断裂是指共价键断开的过程，分为均裂和异裂两种类型。均裂时，共价键平均断裂，每个原子各得到一个电子，形成两个自由基（例如 Cl–Cl → 2 Cl•）。自由基是带有未成对电子的高活性物种。
In heterolytic fission, the bond breaks unevenly: one atom takes both electrons from the shared pair, forming a cation and an anion. For example, H–Br → H⁺ + Br⁻. Heterolytic fission is common in polar reactions and generates ionic intermediates.
异裂时，共价键不均匀断裂：其中一个原子获得共享电子对中的两个电子，形成阳离子和阴离子。例如 H–Br → H⁺ + Br⁻。异裂常见于极性反应，产生离子型中间体。
3. Curly Arrows and Electron Movement | 弯箭头与电子移动
Curly arrows are a universal notation in organic mechanisms. A full-headed curly arrow ( ⟶ ) shows the movement of an electron pair. It starts from a source of electrons – a lone pair or a bond – and points toward an electron-deficient site or the position where a new bond forms.
弯箭头是有机机理中的通用符号。全头弯箭头（ ⟶ ）表示电子对的移动。它从电子源（孤对电子或化学键）出发，指向缺电子位点或新键形成的位置。
A half-headed fish-hook arrow ( ↷ ) is used for single-electron movement, typically in radical reactions. Should you need to show a bond breaking, the curly arrow begins at the centre of the bond and points to the atom taking the electrons.
半头鱼钩箭头（ ↷ ）用于单电子移动，通常出现在自由基反应中。若要表示化学键断裂，弯箭头从键的中间开始，指向接收电子的原子。
4. Electrophilic Addition to Alkenes | 烯烃的亲电加成
Alkenes contain a region of high electron density in the π bond, making them susceptible to attack by electrophiles (electron-deficient species). The general mechanism of electrophilic addition involves initial attack by an electrophile on the double bond, giving a carbocation intermediate, followed by rapid combination with a nucleophile.
烯烃的π键区域电子密度很高，因此容易受到亲电试剂（缺电子物种）的进攻。亲电加成的一般机理是：亲电试剂先进攻双键，生成碳正离子中间体，随后与亲核试剂快速结合。
For example, when HBr adds to ethene, the π electrons attack the partially positive hydrogen, Hδ⁺–Brδ⁻, forming a carbocation CH₃–C⁺H₂ and a bromide ion. This is the rate-determining step. In the second step, the bromide ion acts as a nucleophile and donates its electron pair to the carbocation, forming bromoethane.
例如，HBr与乙烯加成时，π电子进攻带部分正电荷的氢（Hδ⁺–Brδ⁻），生成碳正离子 CH₃–C⁺H₂ 和溴离子，这是决速步。第二步中，溴离子作为亲核试剂，将电子对提供给碳正离子，生成溴乙烷。
Overall equation: C₂H₄ + HBr → CH₃CH₂Br
总反应方程式： C₂H₄ + HBr → CH₃CH₂Br
5. Mechanism: Addition of HBr to Ethene (Step-by-Step) | 机理：HBr与乙烯加成（逐步解析）
Step 1 – Electrophilic attack: The H–Br bond polarises, and the π electrons of C=C form a bond with H⁺. A carbocation CH₃–C⁺H₂ and a Br⁻ ion are generated. Curly arrow from the π bond to H.
第一步 – 亲电进攻： H–Br键极化，C=C的π电子与H⁺成键。生成碳正离子CH₃–C⁺H₂和Br⁻离子。弯箭头从π键指向H。
Step 2 – Nucleophilic capture: The bromide ion uses a lone pair to attack the positively charged carbon, forming a C–Br bond. Curly arrow from Br⁻ to C⁺. The product is bromoethane, CH₃CH₂Br.
第二步 – 亲核捕获： 溴离子利用孤对电子进攻带正电荷的碳，形成C–Br键。弯箭头从Br⁻指向C⁺。产物为溴乙烷CH₃CH₂Br。
If the alkene is unsymmetrical, Markovnikov’s rule predicts the major product.
若烯烃不对称，马尔科夫尼科夫规则（马氏规则）可以预测主要产物。
6. Markovnikov’s Rule and Carbocation Stability | 马尔科夫尼科夫规则与碳正离子稳定性
Markovnikov’s rule states that when a protic acid HX adds to an unsymmetrical alkene, the hydrogen atom attaches to the carbon with the greater number of hydrogen atoms already attached. This is explained by carbocation stability: more substituted carbocations are more stable due to hyperconjugation and inductive effects.
马尔科夫尼科夫规则指出，当质子酸HX与不对称烯烃加成时，氢原子加到原本含氢较多的碳上。这可用碳正离子稳定性解释：取代基更多的碳正离子因超共轭和诱导效应而更稳定。
Stability order: tertiary (3°) > secondary (2°) > primary (1°) > methyl. Therefore, the intermediate that places the positive charge on the more substituted carbon is favoured, leading to the Markovnikov product.
稳定性顺序：叔碳正离子（3°）> 仲碳正离子（2°）> 伯碳正离子（1°）> 甲基碳正离子。因此，正电荷位于取代基更多的碳上的中间体更有利，得到马氏产物。
7. Electrophilic Substitution of Benzene | 苯的亲电取代
Benzene undergoes electrophilic substitution rather than addition because substitution retains the stable aromatic ring. The mechanism requires generation of a powerful electrophile, such as NO₂⁺ in nitration, which attacks the π system. An unstable carbocation intermediate (arenium ion) forms, then loses a proton to restore aromaticity.
苯发生亲电取代而非加成反应，因为取代反应保留了稳定的芳香环。机理需要先生成强亲电试剂，例如硝化反应中的NO₂⁺，它进攻π体系，生成不稳定的碳正离子中间体（芳基正离子），随后失去一个质子以恢复芳香性。
In nitration, concentrated HNO₃ and H₂SO₄ generate NO₂⁺. The overall equation is: C₆H₆ + HNO₃ → C₆H₅NO₂ + H₂O. Curly arrows show the π electrons attacking NO₂⁺ and later the loss of H⁺ with the aid of HSO₄⁻.
在硝化反应中，浓HNO₃和H₂SO₄生成NO₂⁺。总方程式：C₆H₆ + HNO₃ → C₆H₅NO₂ + H₂O。弯箭头表示π电子进攻NO₂⁺，随后在HSO₄⁻帮助下失去H⁺。
8. Nucleophilic Substitution Reactions: SN1 and SN2 | 亲核取代反应：SN1与SN2
Nucleophilic substitution occurs when a nucleophile replaces a leaving group on a saturated carbon. The two limiting mechanisms are SN1 and SN2. In SN2, the nucleophile attacks the carbon at the same time as the leaving group departs, proceeding through a single transition state with inversion of configuration.
亲核取代是指亲核试剂取代饱和碳上的离去基团。两种极限机理是SN1和SN2。SN2中，亲核试剂进攻碳的同时离去基团离开，经单一过渡态，并发生构型翻转。
SN1 is a two-step process: the leaving group first departs to form a planar carbocation, then the nucleophile attacks from either side, leading to racemisation if the carbon is chiral. The rate equation for SN2 is Rate = k[RX][Nu⁻], while for SN1 it is Rate = k[RX] only.
SN1是两步过程：离去基团先离去，形成平面型碳正离子，然后亲核试剂从任一侧进攻，若碳是手性中心则会导致外消旋化。SN2速率方程为 Rate = k[RX][Nu⁻]，SN1则为 Rate = k[RX]。
9. Factors Influencing SN1 and SN2 | 影响SN1与SN2的因素
| Factor | Favours SN2 | Favours SN1 |
|---|---|---|
| Substrate structure | Primary > secondary > tertiary (less steric hindrance) | Tertiary > secondary (stable carbocation) |
| Nucleophile strength | Strong, concentrated nucleophile required | Nucleophile strength less critical; weak nucleophile may be used |
| Leaving group | Good leaving group aids both, but essential for SN2 | Excellent leaving group promotes carbocation formation |
| Solvent | Polar aprotic solvents (e.g., acetone) enhance nucleophilicity | Polar protic solvents (e.g., water, ethanol) stabilise ions |
Understanding these factors allows you to predict which mechanism will dominate under given conditions. For instance, a tertiary alkyl halide with a weak nucleophile in a polar protic solvent will favour SN1.
理解这些因素能让你预测在给定条件下哪种机理占主导。例如，叔卤代烷在极性质子溶剂中与弱亲核试剂反应倾向于SN1。
10. Elimination Reactions: E1 and E2 | 消除反应：E1与E2
Elimination reactions form alkenes by removing atoms or groups from adjacent carbon atoms. In the E2 mechanism, a strong base removes a β-hydrogen at the same time as the leaving group departs, in a single concerted step. This requires an anti-periplanar arrangement of H and the leaving group.
消除反应通过从相邻碳原子上脱去原子或基团而生成烯烃。在E2机理中，强碱夺取β-氢的同时离去基团离开，是一个协同过程，要求氢与离去基团处于反式共平面。
In E1, the leaving group first leaves to give a carbocation, which then loses a β-proton to a base. The reaction is stepwise and competes with SN1. E1 reactions typically occur with tertiary substrates under neutral or weakly basic conditions.
在E1中，离去基团先离去生成碳正离子，然后失去一个β-质子给碱。该反应分步进行，与SN1竞争。E1通常发生于叔碳底物，在中性或弱碱性条件下进行。
Example: 2-bromopropane with hot ethanolic KOH gives propene via E2: CH₃CHBrCH₃ + OH⁻ → CH₃CH=CH₂ + Br⁻ + H₂O.
示例： 2-溴丙烷与热的乙醇KOH溶液通过E2生成丙烯：CH₃CHBrCH₃ + OH⁻ → CH₃CH=CH₂ + Br⁻ + H₂O。
11. Free Radical Substitution of Alkanes | 烷烃的自由基取代
Alkanes are relatively unreactive but undergo free radical substitution with halogens in the presence of UV light. The mechanism proceeds in three stages: initiation, propagation, and termination. Initiation involves homolytic fission of Cl₂ into two chlorine radicals (Cl•) using energy from UV light.
烷烃相对不活泼，但在紫外光照射下可与卤素发生自由基取代。机理包括三个阶段：引发、增长和终止。引发阶段利用紫外光能量使Cl₂均裂为两个氯自由基（Cl•）。
In propagation, a chlorine radical abstracts a hydrogen atom from methane, forming HCl and a methyl radical (CH₃•). The methyl radical then reacts with another Cl₂ molecule, producing chloromethane and regenerating a chlorine radical, sustaining the chain reaction.
在增长阶段，氯自由基从甲烷中夺取一个氢原子，形成HCl和甲基自由基（CH₃•）。甲基自由基再与另一个Cl₂分子反应，生成氯甲烷并再生氯自由基，使链反应持续。
Termination occurs when two radicals combine, e.g. 2 Cl• → Cl₂, or Cl• + CH₃• → CH₃Cl. Further substitution can lead to a mixture of products.
终止反应发生在两个自由基结合时，例如 2 Cl• → Cl₂ 或 Cl• + CH₃• → CH₃Cl。进一步取代会生成混合物。
12. Nucleophilic Addition to Carbonyl Compounds | 羰基化合物的亲核加成
Carbonyl compounds (aldehydes and ketones) have a polar C=O bond, making the carbonyl carbon electron-deficient and susceptible to attack by nucleophiles. In the addition of HCN, the cyanide ion CN⁻ acts as a nucleophile, attacking the carbonyl carbon and pushing the π electrons onto oxygen, forming an alkoxide intermediate.
羰基化合物（醛和酮）具有极性C=O键，使得羰基碳缺电子，易受亲核试剂进攻。在HCN加成反应中，氰根离子CN⁻作为亲核试剂，进攻羰基碳，将π电子推向氧原子，形成醇盐中间体。
Protonation of the alkoxide yields a hydroxynitrile (cyanohydrin). For ethanal: CH₃CHO + HCN → CH₃CH(OH)CN. The reaction is catalysed by base, which generates CN⁻ from HCN. This mechanism is a classic example of nucleophilic addition.
醇盐质子化后得到羟腈（氰醇）。以乙醛为例：CH₃CHO + HCN → CH₃CH(OH)CN。反应由碱催化，碱从HCN生成CN⁻。该机理是亲核加成的经典例子。
The rate-determining step is the attack of the nucleophile, and the carbonyl group is planar, so the nucleophile can attack from either face, often leading to a racemic mixture if a new chiral centre is formed.
决速步是亲核试剂的进攻，且羰基是平面型的，亲核试剂可从两面进攻，若形成新的手性中心，通常会得到外消旋混合物。
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